Can anyone help me with Molarity question?

Question

I have two solutions. In the first solution, 1.0 moles of sodium chloride is dissolved to make 1.0 litre of solution. In the second one, 1.0 moles of sodium chloride is added to 1.0 litre of water. Is the molarity of each solution the same? Why or why not?

Answer 1

There is more to this question than meets the eye. You know immediately that the first solution is 1M because there is one mole of  NaCl in one litre of solution. But what about the second solution?

Well, you added 58.5 g NaCl (density 2.16 g/cm^3) to 1L H2O, so you would expect the total volume to be 1L + 58.5/2.16 cm^3 = 1027ml and you would calculate the molarity as 1/1.027 = 0.974M.

This would be wrong. The combined volume of salt and water is LESS that the sum of their separate volumes (Why?), so the molarity of your second solution will be greater than 0.974M. I estimate it will be close to 0.98, but you will have to do the experiment to find out exactly....

http://www.practicalphysics.org/go/Exper...

Answer 2

Technically, no, they aren't the same.   The second solution will have a slightly lower molarity once you account for the volume of the dissolved NaCl.

However, compared to molarity changes due to temperature (i.e. thermal expansion/contraction of the solvent) the difference is negligible.

Are you sure the question isn't asking about molality (mol/kg solvent) and not molarity (mol/L solution)?  There's about a 5% difference in molality.

Answer 3

They will not have the same molarity.

In the first example 1 mole of NaCl will be present in 1 litre of solution.

In the second example, because you started with 1 litre of water, the final volume of solution will be considerably greater than 1 litre.

So the second solution will be the more dilute.

Remember the molarity (concentration) of a solution is the number of moles of solute per dm3 (litre) of SOLUTION.

Answer 4

CB's answer is correct .

Quick correction to one of the other answers.

Molality in the moles of solute / kg of solvent, not litres.

Answer 5

They will be the same. Molarity is calculated by moles of solute *divided by* liter of solution. (Water is a solution in this case.) One mole of salt in one liter of solution will cause a molarity of one, as will one mole of salt in one liter or water.

Answer 6

Yes, its the same. Molarity is defined as the no of moles solute per liter solvent.  

Answer 7

No the molarity is not the same.

Molarity is Moles of NaCl per liter of SOLUTION (first solution)

Molality is Moles of NaCL per liter of SOLVENT (second solution)

The molarity of the second solution would be less than the molarity of the first solution, because there is more solvent in the second solution than the first solution